It is a trend to increase the strength and reduce the weight of all kinds of transportation vehicles (bikes, motorbikes, cars, trucks, aircraft, space shuttle and others). With reduced weight, reduced fuel consumption and reduced gas emissions to follow. Today, automobile manufacturers are using more and more plastics, but plastic's strength to weight ratio is low when compared to light metals like aluminum, and in particular magnesium. Plastic also has the disadvantage of being difficult to recycle and separate it from other materials in automobiles. Light alloy parts in cars are easy to separate for recycling and materials are generally environmentally friendly with lower energy impact.
Prior art in the field of light alloy castings is based on the premise that a melting pot is required for melting material, after which the molten material is transported into the die-casting machine. Basically, in prior art, the die-casting process is accomplished by melting material in big pot, transferring the material into a machine (manually or by robot) and injecting this molten material into a cavity with high force and low to high speed. Considering the fact that in prior art molten material resides in a big pot, it is a requirement of the process that the molten material is overheated (superheated). For magnesium this melt temperature is 700°-780° C. Superheated melting is done to overcome cooling losses encountered in the process of melt transfer from pot to the die-casting machine. Intense energy requirements for this process are a major drawback for this technology. Furthermore, handling the melt in the manufacturing process is riddled with losses and melt contamination. Intense oxidation of the melt results in poor castings. Injection of material into the cavity requires high speed, and turbulence from the process often results in extensive inclusions in the castings. Defects of this nature are detrimental for applications in the automotive industry, particularly for castings related to vehicle safety. From the above brief description of the current state of the art we can see a need for more efficient machines that will reduce energy consumption to a minimum and totally eliminate Green House Gas (GHG) use.
Die-casting is a manufacturing process used to produce a part in near-net shape with high dimensional accuracy and a good surface finish in a short cycle time. The casting industry branched in two directions: Melt processing, where hot and cold chamber casting dominate, and semi solid slurry processing where Rheomolding and Thixomolding® routes have been adopted. Cornell research foundation's U.S. Pat. No. 5,501,266 discloses a process called Rheomolding. Superheated liquid metal supplied from outside is cooled into a semi-solid state in the barrel of a special vertical-injection molding machine, with the growing dendrites of the solid state broken into small and nearly spherical particles by the shearing force generated by the screw and barrel. It was said that this process can produce net-shape metal parts at a lower cost but this has not been the case under real market conditions. These machines are very expensive and complex, difficult to operate and support. The Rheomolding route has not been often used. The Thixomolding® route, also known as semi-solid casting or molding (as terms used in the plastics industry) has been more widely adopted.
Conventional die casting apparatuses are classified into cold chamber and hot chamber. The cold chamber die-casting process uses a superheated molten metal alloy. Referring to FIG. 14 we can see a cold chamber die casting machine. Molten alloy (magnesium, aluminum or zinc) is injected into a closed metal die under high pressure by way of a high-speed ram. The alloy is driven through the feed system of the die, while air from the mold escapes through vents. There must be enough metal to overflow the cavity, such that a complete part will be cast. Once full, the injection pressure on the mold is increased during solidification. The pressure is increased during solidification to reduce porosity due to shrinkage. To complete this path from the molten pot to the die cavity without starting to solidify, the melt must be superheated up to 100° C. above liquidus temperature. As the metal dies (or molds, as they are known in the plastics industry) are cooled, molten metal gets solidified into a predetermined shape. Once sufficiently cooled, the part is removed from the die.
The second well known process for casting light metal alloys is the hot chamber die casting method. Referring to FIG. 15, we can see a hot chamber die casting machine. The pressure chamber (cylinder) and the plunger are submerged in the molten metal in the pot (crucible). Hot chamber die casting means, compared to cold chamber, that the molten metal is transported directly into the die via a heated channel called a “gooseneck”, thus minimizing heat loss.
As one can appreciate, both of the above-mentioned processes use melt that is heated to higher than optimal casting temperatures to compensate for heat losses. Hot chamber die-casting does not require the melt to be as hot as in cold chamber. To reduce heat losses of the melt, a significant portion of the injection system is submerged in molten metal at all times. The benefit of hot chamber die-casting is that melt travels a short distance and the cycle time is reduced. However, high temperature and continued exposure to aggressive melt creates severe material deterioration problems. As is well known, both processes suffer poor reliability due to lack of suitable materials for melt containment and no means to overcome melt corrosion and high pressure and high applicable temperature. Both processes suffer from material shrinkage in the cast parts, from 5-15%. High injection rates also cause gases to be mixed into the melt and becoming trapped in the part. Porosity is the biggest problem for a part's structural integrity. Molded metallic parts with high porosity are not desirable because of their reduced mechanical strength. It is because of this that it is very difficult to accurately dimension conventionally die cast parts, and it is even more difficult to maintain the dimensions throughout life cycle of the part. Therefore, the quality of the components made on these machines is generally poor and often does not meet the stricter requirements for the automobile industry. Because the scrap rates are high, die casters continue to use melt pots, as this allows the immediate remelting of the scrap parts. Unfortunately, producing scrap still requires energy to remelt the part, and cover gases, such as sulfur hexafluoride (SF6) and carbon dioxide (CO2) are wasted. Both gases have a significant environmental impact.
Besides environmental pollution, cast parts made from super heated melt are often plagued by entrapped porosities and inclusions created by large amounts of shrinkage due to rapid material cooling from superheated melt to solid near net shape parts.
FIG. 16 shows an injection molding apparatus adopted from thermoplastic processing. This apparatus has a composite cylinder with an inner diameter of 50 to 200 mm and a length of approximately 2 to 5 m. A specially devised drive is coupled to a retractable helical screw designed to transport the alloy material along the cylinder. The heat to melt the metal alloy is provided by a series of heated zones arranged along the cylinder. The forward end of the cylinder is closed by the cylinder cover but allows material transfer into a nozzle portion at the distal end of the cylinder. A specially designed check valve is placed at the forward end of the screw to facilitate injection of the molten slurry into the mold. In the art of plastic injection molding the cylinder is called a barrel and whole assembly is well known as an extruder. The cylinder can be a monolithic tube or made from Inconel 718 with specially fitted Stellite liner to reduce corrosion. Stellite is a Cobalt alloy with specific corrosion and abrasion properties suitable to contain and convey molten magnesium.
In this process, solid chips of alloy material are supplied to the injection molding apparatus through a feeder portion often called a hopper. The size of the chips is approximately 2-3 mm in diameter and generally is no longer than 10-12 mm. The chips are produced from standard die casting alloys in ingot form. The ingots are chipped to size by a separate machine designed for this purpose. The comminuted chips are fed into a hopper and further processed in the injection molding extruder into a supposedly preferential state called a slurry-like melt, which is, in its best form, in a partially molten state. The injection screw shears the melt and pushes the melt forward over a check valve on the distal end of the extruder and is subsequently injected into a closed and clamped injection mold. The machine nozzle dispenses the thixotropic slurry into a mold portion of the SSIM apparatus, often called a sprue. The sprue is a part of the mold assembly not described in this enclosure.
There is a clear advantage of the slurry (Thixomolding®) process over the die casting process in the fact that process does not use SF6 cover gas. Small amounts of argon gas are used to protect the melt from oxidation. Argon is heavier than air and tends to stay close to earth and gets dissolved and returned to air naturally. However, one familiar with this state of the art will appreciate that the Thixomolding® and similar semi-solid processes are complex and require a very long melt passageway. All of these methods and processes are carried out within a single cylindrical housing. Manufacturing suitable barrels is a tedious and requires expensive alloys and processes. As a result only a few suppliers are able to produce composite barrels with Stellite liners in Inconel housings with the dimensional requirements for large throughput for any serious part molding using these methods.
Very accurate control of the process temperature is essential for successful and repeatable molding of good parts with injection molding methods disclosed above. It is very difficult to control all of the process parameters within a single cylindrical housing, particularly temperature, shot volume, pressure, cycle time, etc., and as a result, inconsistent characteristics of the molded metallic parts are produced. As a consequence, if a molded metallic part of undesired characteristics is produced by a semi solid slurry molding machine, recycling of the defective part is not possible. Metal parts molded by injection molding machines with high solid contents may have an uneven surface. Such metal parts may require further processing before they can be painted. Finally, the above mentioned injection molding process is complex and expensive to manufacture, and is plagued by the reliability of its machine parts. Further, it lacks the wide operating window and stability that are required for a viable manufacturing process.
One skilled in the art can recognize the complexities involved in the die casting process and structures (cold and hot chamber) as well as in the molding process and structures (Rheomolding and Thixomolding®). Both processing routes are largely unreliable and suffer from a lack of consistency from shot to shot and part to part. There is a need for a new and simpler structure with a stable processing window and without the use of SF6 cover gas. Furthermore, molds for above machines are mostly cooled by oil. Oil is environmentally unfriendly and there is a need to eliminate the oil for any kind of cooling on the machine.